Enable Dark Mode in OS X Yosemite Beta with a defaults Command
Mac users running OS X Yosemite Beta 2 can get a glimpse at the upcoming “dark mode” feature that was mentioned briefly during the Yosemite overview presentation during WWDC 2014. Essentially, dark mode turns many user interface elements of OS X Yosemite into significantly darker variations of themselves, swapping out whites and light grays for blacks and dark grays. With OS X 10.10 Developer Preview 2, this darkening effect is limited to menubar items and the Dock, but it’s possible the visual effect will reach the broader window coverage and theme of OS X Yosemite as well.
If you’re currently running the most recent version of OS X 10.10, you can try the incomplete dark mode effect out yourself by using a defaults write command.